Digital cameras and scanners create image information by converting light into electrical signals using a photo-sensor. Typically the photo-sensors are charged coupled devices (CCD). The CCD's typically have 3 color filters arranged over the surface to sample the light in three different colors. Typically these are red, green, and blue. Scanners typically use an internal light source to illuminate the page. Cameras may use only the ambient illumination or they may add illumination to the scene using a flash. The electrical signal created by each pixel in the CCD is a function of the illumination in the scene, the reflectance of the object in the scene, the pass band of the filter over the pixel, and the inherent sensitivity of the CCD to light. One of the goals of these systems is to reproduce the color of an object on a computer display and with printed output. The computer display typically has different output characteristics than printers. One method used to help match the display with printers is call tone mapping. U.S. Pat. No. 5,719,965 entitled “Synthesized higher precision look-up table from lower-precision look-up table” which is hereby incorporated by reference, describes how tone maps are used. One type of adjustment done with tone maps is “gamma correction”. Gamma correction typically maps reflectance space with a power curve. This curve has the property of increasing the contrast of the image in the dark areas and decreasing the contrast in the light areas. The tone map operation can be done in the scanner or camera hardware, or the raw color data may be sent to a computer where the image processing (including the tone map operation) is done.
CCD's sample light and convert it into electrical signals. These electrical signals are then converted into digital information typically using an A-to-D converter. CCD's are typically fairly linear in response to the intensity of light falling on each pixel. The human eye is not linear in response to intensity. The human eye responds more to density, which is the log of intensity. Because of this phenomenon a given change in intensity will be much more noticeable to the human eye if the change occurs in a dark area as compared to a light area of an image. For example in a system with 8 bits of resolution or 256 levels of intensity and 0 as black and 255 as white, a 2-count change in intensity from 10 to 12 may be much more noticeable to the human eye than the same 2-count change in intensity from 200 to 202.
The number of levels of light that the CCD and A-to-D converters can create has increased in recent years. Older scanners and cameras typically had 8 bits per color (256 levels) for a total of 24 bits. Today CCDs and A-to-D converters create 10, 12 and even 16 bits per color. In addition to the increase in the number of levels, currently available CCDs have improved their signal-to-noise ratios. This means that more of the total number of levels are actual signal instead of noise. These changes have made reproductions of the dark areas of the images much more accurate.
Unfortunately these changes have also created some problems. One of these problems is the tone map operation. In the dark areas of the image, inaccuracies in the tone map operation may show up as slight unintentional color changes in the image. These inaccuracies in the dark area are exaggerated when the tone map is using a gamma correction curve. The human eye may be unable to detect a slight change in brightness, but the human eye is very sensitive to small color shifts in the dark areas of an image. These small changes in color in the dark area of images can reduce the usability of these images.
What is needed is a method and apparatus that reduces the inaccuracies in the tone map operation in the dark areas of the image.